1. Field of the Invention
The present invention relates to an acceleration detecting device for detecting the acceleration applied to moving contacts made integral with a mass body contacting with stationary contacts while the mass body moves a predetermined distance. In particular, the acceleration detecting device is applicable on a moving body such as an automobile, used for starting up an air bag, side air bag, seat belt pretensioner, or the like when the moving body makes collision.
2. Description of the Related Art
An acceleration detecting device of this type is disclosed in, for example, Japanese Patent Laid Open Hei-9-211023. As shown in FIG. 11, a mass body 1 is made unitary with moving contacts 3a, 3b, and an elastic member (coil spring) 4 biases the mass body 1 along a slide shaft 10 on which the mass body 1 slides, in a predetermined direction indicated by an arrow 11. When the mass body 1 is moved against the bias of the elastic member 4 upon application of acceleration in a direction opposed to the arrow direction 11, the moving contacts 3a, 3b made unitary with the mass body 1 contact, as shown in FIG. 12, with stationary contacts 5a, 5b provided on the inner peripheral surface of a casing 2 to thereby turn on electricity, and a signal as shown in FIG. 13 is sensed whereby an acceleration of a predetermined magnitude or more is detected.
In this acceleration detecting device, in the case where a high acceleration is applied, causing the mass body 1 to collide against the distal end of the space in the casing 2 covering the device in which the mass body 1 can move, fluctuations of the moving contacts 3a, 3b caused by the shock temporarily make the state of contact between the moving contacts 3a, 3b and the stationary contacts 5a, 5b unstable, resulting in the signal shown in FIG. 14, so that the period of conduction is insufficiently long. This complicates the judgment processing for starting up an air bag or the like.
In order to solve the problem, fluctuations of the moving contacts 3a, 3b upon application of a high acceleration were prevented conventionally by providing a shock absorbing part 12, which is formed of a material such as rubber, on the area where the mass body 1 collides against the casing 2 to relieve the shock caused at the time of collision and decelerates the mass body 1, whereby it is intended that sufficient conduction time is ensured.
However, a shock absorbing material such as rubber, possesses a property that the degree of shock absorption varies due to temperature-dependent change in hardness. Thus a new measure such as an increase in the whole length of the device is necessary to ensure conduction time in the temperature range in which it must be guaranteed that a moving body such as an automobile can operate.
The present invention has been conceived in order to solve the above problems, and its object is to provide an acceleration detecting device in which shock absorbing parts are omitted, and in order to ensure reliable conduction even in the case where a high acceleration is applied to a mass body causing it to collide against the casing portion at the distal end of the range in which the mass body moves, a construction for providing for auxiliary conduction is added to ensure a stable conduction time unaffected by ambient temperature or the like.
In order to attain the object, the invention provides an acceleration detecting device comprising a mass body inserted in a casing and movable in a forward and backward direction, an elastic member for biasing the mass body backward, stationary contacts provided on the casing, moving contacts provided on the mass body, and auxiliary moving contacts provided on the mass body, provided so that the moving contacts contact with the stationary contacts when acceleration applied to the mass body causes the mass body to advance a predetermined distance against the bias of the elastic member, and when the mass body advances further and collides against the casing at the distal end of range in which the mass body moves, the auxiliary moving contacts contact with the stationary contacts.
Accordingly, since the time of conduction through the contacts is stabilized and a member such as rubber for relieving the collision between the mass body and a casing can be omitted, it is possible according to the invention to obtain a highly reliable acceleration detecting device.
The other objects and features of the invention will become understood from the following description with reference to the accompanying drawings.